To calculate the gradient/slope of a linear graph/line:
(y(1) - y) / (x(1) - x)
NOTE: the (1) doesn't mean anything, I'm just showing you that x(1) and y(1) comes from a different point from x and y
In this case:
gradient of line = (9 - 1) / (3 - 1)
= 4
Therefore the answer is D